The iron based blended powder for powder metallurgy is formed by blending an iron powder with a Cu powder or carbon powder, compacted in a die, and then sintered into a sintered body having ordinarily a density of 5.0 to 7.2 g/cm.sup.3 to be used as mechanical parts and so forth. In order to improve the sliding property of a sintered article that is to be used as mechanical parts or the like, such sintered steel as contains free graphite like cast iron is considered effective.
For example, Japanese Patent Laid-Open Publication No. Hei 8-209202 has suggested such a blended powder that provides a sintered article which contains a maximum of 0.5% by weight of free graphite therein and has an improved sliding property. The blended powder is formed by blending an iron power with a graphite powder, the iron powder containing B, Cr, and Mn, as well as one or more elements selected from the group consisting of S, Se, and Te, and a partially alloyed element selected from the group consisting of Ni, Cu, and Mo.
Furthermore, Japanese Patent Laid-Open Publication No. Hei 8-144,026 discloses a structure, of a free graphite precipitate iron based sintered article with high strength and a high toughness, containing C, Ni, Mo, Cu, BN, and S with the residue Fe and unavoidable impurities, said BN being distributed in the boundary faces of said polycrystalline bodies.
However, recently, various types of drive units for use in automobiles or the like have increasingly been required to provide higher output and reduced weight, leading to more extreme conditions for the sliding parts to be used therein, so that sintered articles are required to have still further improved sliding properties.
However, in order to increase the content of free carbon in a sintered article to 0.5% by weight or more, merely increasing the quantity of carbon to be blended into an iron based blended powder for powder metallurgy as set forth in Japanese Patent Laid-Open Publication No. Hei 8-209202 would cause the sintered article to increase in brittleness due to excessive carbonization, thus raising such problems that the impact resistance is reduced and the sintered article cannot be reformed. Furthermore, merely increasing the content of the carbon added to said iron based blended powder for powder metallurgy would cause the carbon to segregate from the iron powder due to the difference in their specific gravity during the transportation of the blended powder or at the time of the feed of the same, thus raising such a problem that sintered articles have variations in sliding properties.
Furthermore, the technique set forth in Japanese Patent Laid-Open Publication No. Hei 8-144026 was intended to obtain as high a toughness as the present invention, however, it has disclosed no data on the sliding property.